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    <title>Document</title>
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    <script>
        //原型对象的应用 扩展内置对象方法
        console.log(Array.prototype);
        Array.prototype.sum = function(){
            var sum = 0;
            for(var i =0;i<this.length;i++){
                sum +=this[i];
            }
            return sum;
        }
        var arr = [1,2,3];
        console.log(arr.sum());
        console.log(Array.prototype);
        //在数组和字符串内找对象不能给原型对象覆盖操作Array.prototype ={}，
        //只能是Array.prototype.xxx = function(){}的方式
    </script>
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